The Sun, 20 mph speed limits, and a lot of nonsense

A report from the Department for Transport shows there were 2,262 fatalities and injuries in 2011 on 20mph roads in built-up areas — 24 per cent more than in 2010 when 1,827 were recorded.

That compares to a one per cent reduction in casualties on 30mph roads in built-up areas.

How many people died or were injured on roads with a 30 mph speed limit then? The Sun doesn’t say, but the official data is easy enough to find. In 2010, 127,377 people were killed or injured on a road with a 30 mph speed limit. In 2011, it was 125,494.

In other words, that “1% drop” corresponds to almost 2000 fewer people were injured on 30 mph roads, while 400 more people were injured on 20 mph roads. That is a net decrease of 1448 injuries/deaths.

Just looking at the number of injuries on 20 mph roads is not going to tell you about how effective 20 mph speed limits are. You have to compare them with other speed limits. Data about how many 20 mph zones there are is hard to find, which makes making sense of the Sun‘s data difficult, but it certainly looks like a lot of them have been springing up lately, so an increase in injuries is to be expected – if there are more roads, there will always be more accidents!

Studies of 20 mph zones consistently find that the same stretch of road will see fewer accidents and fewer injuries if the speed limit decreases from 30 mph to 20 mph, especially if this decrease is enforced by speed bumps and other traffic calming measures.

* Incidentally, far from being “controversial”, surveys consistently find around 74% in favour of 20 mph speed limits and just 12% against them.

Another statistic missing from the Sun’s “report” is the speed of the vehicles involved in the incidents on 20mph roads. My own experience suggests that (unenforced) a 20mph limit succeeds in reducing the speed of most cars passing through it to around 30mph.
This would perhaps be covered in the “false sense of security” that is mentioned – pedestrians are expecting cars to be travelling at or below the speed limit, when in fact they are travelling 50% above it.